The detection of currents in DC circuits is associated with greater technical problems than such detection of currents in AC circuits, in which transmission to a measurement device or to a tripping circuit of a switching device can be carried out using magnetic transformers.
Furthermore, there is the requirement for low-voltage switchgear for the measurement to be carried out as far as possible from the system, that is to say without providing external energy from an additional energy source which provides an auxiliary voltage. This is also only a minor problem in AC networks, or at least with an energy source having only a low power level.
It is known for the primary current to be measured using magnetic devices, that is to say with the aid of Hall probes or magnetoresistive sensors. However, it has been found that this method is not very suitable, at least for the preferred situation here, since the primary current cannot be reproduced with sufficient accuracy. As such, spurious tripping of the overcurrent release can thus easily occur.
It is also known for a measurement resistor (shunt) to be inserted in the primary circuit and for the voltage to be supplied via this measurement resistor to an isolating amplifier, whose output side acts on the tripping circuit. However, this solution is technically highly complex owing to the requirements which the isolating amplifier has to satisfy. Furthermore, there is a continuous, high power loss in the measurement resistor.
EP-A 0 651 258 discloses a measurement method for direct currents, in which the primary conductor is magnetically coupled to a field winding via an iron core. The latter is energized with a regular, triangular-waveform alternating current, which allows pulses to be produced at regular intervals in a further winding, which is used as a measurement coil. If the iron core is now premagnetized in one direction or the other by means of the primary direct current, then the shift in the hysteresis curve of the iron core changes the interval between the measured pulses, and this can be evaluated as a measure of the primary current to be measured. This solution is likewise very complex in terms of circuitry and requires the continuous provision of an auxiliary voltage with a corresponding power level, thus resulting in a not inconsiderable energy requirement.
A further method for measuring direct currents is based on the use of a field winding to compensate for a magnetic field caused by the direct current in an iron core. The magnetic field in an air gap in the iron core is measured using a magnetic field sensor, in order to control the compensation current in the field winding. The current flowing in the field winding once compensation has been carried out, that is to say once the total field measured by the magnetic field sensor has become zero, is used as a measure of the primary direct current. The method is known, for example, from EP-A 0 294 590 or from DE-A 38 15 100.
A further option with the aid of magnetic field compensation is described in UK-A 2 029 973. According to this solution, a current in the form of a ramp is fed continuously and repeatedly into the compensation winding and the magnetic field in the iron core is measured via an indicator winding.
The compensation method likewise requires a considerable amount of energy to provide a continuous compensation current, and is thus not suitable for the above-mentioned purpose.